The invention includes a method for controlling metal dusting corrosion in reactor materials exposed to carbon supersaturated environments and a composition of matter.
In many hydrocarbon conversion processes, such as, for example the conversion of CH4 to syngas, environments are encountered that have high carbon activities and relatively low oxygen activities. High temperature reactor materials and heat exchanger materials used in such processes can deteriorate in service by a very aggressive form of corrosion known as metal dusting.
Metal Dusting is a very deleterious form of high temperature corrosion experienced by Fe, Ni and Co-based alloys at temperatures in the range, 400-900xc2x0 C. in carbon-supersaturated (carbon activity greater than 1) environments having relatively low (about 10xe2x88x9210 to about 10xe2x88x9220 atmospheres) oxygen partial pressures. This form of corrosion is characterized by the disintegration of bulk metal into metal powder. Although many commercial alloys are available that are designed to form protective Cr2O3 or Al2O3 films in low oxygen partial pressure environments, the nucleation and growth kinetics of these oxides are often not fast enough to block carbon intrusion in environments with carbon activities in excess of unity.
Methodologies available in the literature to control metal dusting corrosion involve the use of surface coatings and gaseous inhibitors, especially H2S. Coatings can degrade by interdiffusion of the coating constituents into the alloy substrate. Thus, while coatings are a viable approach for short-term protection, they are generally not advisable for long term service life of twenty years or more. Inhibition by H2S has two disadvantages. One is that H2S tends to poison most catalysts used in hydrocarbon conversion processes. Furthermore, H2S has to be removed from the exit stream which can substantially add to process costs.
What is needed in the art is an alloy composition that is resistant to metal dusting corrosion in low (about 10xe2x88x9210 to about 10xe2x88x9220 atmospheres) oxygen partial pressure and carbon-supersaturated (carbon activity greater than 1) environments.
The invention includes a composition of matter which is resistant to metal dusting and comprises (a) an alloy capable of forming a protective oxide coating on its surface when exposed to a carbon supersaturated environment, (b) a protective oxide coating comprising at least two layers on said alloy surface which are formed when said alloy is exposed to metal dusting environments with low oxygen partial pressures. The outer layer, also referred to as the first layer (the layer contacting the carbon supersaturated environment or furthest away from the alloy) is made up of a thermodynamically stable oxide, which can rapidly cover up the alloy surface and block carbon entry into the alloy. The first layer is a thermodynamically stable manganese oxide which forms faster than the carbon in the supersaturated environment is able to penetrate the surface of the alloy. Hence the manganese oxide is referred to as a fast forming layer. Beneath the manganese oxide layer, a second layer forms (herein referred to as said second oxide layer) either simultaneously with or following said manganese oxide formation. The second layer of the protective oxide coating is an oxide film which is established beneath the manganese oxide layer and adherent to the manganese oxide layer and its composition is dependent on the composition of the alloy from which it is formed. Hence the invention includes a composition resistant to metal dusting corrosion comprising (a) an alloy and (b) a protective oxide coating on said alloy, wherein said protective oxide coating comprises at least two oxide layers, wherein the first oxide layer is a manganese oxide layer and wherein said alloy comprises alloying metals and base metals, said alloying metals comprising a mixture of chromium and manganese and said base metals comprising iron, nickel and cobalt, and wherein said manganese is present in a concentration in said alloy of at least about 10 wt % Mn and said chromium is present in said alloy at a concentration of at least about 25 wt % Cr and wherein the combined amount of chromium and manganesexe2x89xa740 wt % and wherein said first oxide layer is the layer furthest away from said alloy surface.
The protective oxide coating may be formed in situ during use of the alloy in a carbon supersaturated environment, or prepared by exposing the alloy to a carbon supersaturated environment prior to the alloys use. A benefit of the invention is that if the protective oxide coating cracks during use of the alloy in a carbon supersaturated environment, the protective coating will form in the crack to repair the oxide layers thereby protecting the alloy from metal dusting during use.
The invention also includes a method for preventing metal dusting of metal surfaces exposed to carbon supersaturated environments comprising constructing said metal surface of, or coating said metal surfaces with a metal dusting resistant alloy composition comprising a metal alloy comprising alloying metals and base metals, said alloying metals comprising a mixture of chromium and manganese and said base metals comprising iron, nickel and cobalt, and wherein said manganese is present in a concentration in said alloy of at least about 10 wt % Mn and said chromium is present in said alloy at a concentration of at least about 25 wt % Cr and wherein the combined amount of chromium and manganesexe2x89xa740 wt % and wherein said first oxide layer is the layer furthest away from said alloy surface.
The metal surfaces may be constructed of the alloy or coated with the alloy and the protective oxide film described above will be formed in situ during operation of the unit in a carbon supersaturated environment.
Hence the invention further comprises a protective oxide coating comprising at least two oxide layers wherein said first layer is a manganese oxide layer and said first layer is the layer furthest away from said alloy on said alloy.